1. Field of Invention
This invention is related to microelectromechanical systems (MEMS).
2. Description of Related Art
A common step in fabricating microstructures is a wet etching step to release a portion of the microstructures from a substrate. The etching step forms a xe2x80x9csuspendedxe2x80x9d microstructure having a void or gap between the released portion of the microstructure and the substrate. The released portion of the microstructure is typically a beam or plate having top and bottom surfaces which are suspended substantially parallel with the surface of the substrate. Common suspended microstructures include cantilevered beams, double supported beams and plates suspended above a substrate by four supports. Devices which incorporate such suspended microstructures include accelerometers, pressure sensors, flow sensors, transducers, microactuators, and electrostatic comb drives.
One known method of forming suspended micromachined microstructures is generally termed surface-micromachining. Surface-micromachining involves additive forming of the microstructure over a substrate. For example, a sacrificial oxide layer, such as silicon dioxide, is deposited over the surface of a substrate of a wafer. The sacrificial oxide layer is selectively etched partially or completely through to the substrate to open up holes in the sacrificial oxide layer.
A thin film microstructure material, such as polysilicon, is deposited over the sacrificial layer. The microstructure material fills in the holes where the sacrificial layer was etched down to the substrate and contacts the substrate to form anchors for supporting the microstructure. The microstructure also fills in the holes where the sacrificial layer was not completely etched down to the substrate to form bumps on the bottom surface of the microstructure. Enough microstructure material is deposited to fill in completely the holes, as well as to form a uniform layer over the top of the sacrificial layer.
The microstructure material is then patterned into a desired shape by photolithography. Finally, the sacrificial layer is removed by, for example, wet etching, leaving behind a microstructure suspended above the substrate by the anchors.
In the micromachining process, the released portion of the microstructures often permanently adhere to the substrate after post-etch rinsing and drying procedures. This microstructure adhesion phenomenon is commonly referred to as stiction. Stiction reduces the micromachining process yield. The bumps on the bottom surface of the microstructure prevent stiction by preventing the microstructure from falling down onto the substrate.
Microstructures are often formed on silicon-on-insulator (SOI) structures. A silicon-on-insulator structure typically includes a silicon substrate, a buried oxide layer formed on top of the silicon substrate, and a single crystal silicon And (SCS) layer formed on top of the buried oxide layer. Forming microstructures on silicon-on-insulator structures provides significant advantages, such as superior electrical isolation between adjacent components, reduction of integrated circuit capacitance, and lower operating voltages.
Silicon-on-insulator structures are provided with the microstructure material already formed on the upper layer and the buried oxide layer formed as a continuous film below the microstructure material. Thus, conventional methods of forming microstructures having anchors and bumps can not be directly implemented.
It is possible to increase the size of the non-floating xe2x80x9canchorxe2x80x9d such that some buried oxide remains intact during a timed selective buried oxide layer etch. However, this process significantly reduces device density. This is disadvantageous, because many emerging micro-devices require numerous, tightly-spaced, moving parts.
This invention provides methods of forming high density microstructures on silicon-on-insulator wafers.
This invention separately provides methods of forming dimples on single crystal silicon structures built on silicon-on-insulator wafers and structures incorporating such dimples.
This invention separately provides methods of forming anchors on single crystal silicon structures built on silicon-on-insulator wafers and structures incorporating such anchors.
Various exemplary embodiments of the methods, and the resulting structures, according to this invention comprise forming at least one opening in a structural layer of a semiconductor structure, forming an opening in a sacrificial layer of the semiconductor structure below the at least one opening in the structural layer, filling the opening in the structural layer and the opening in the sacrificial layer with a filler material, removing at least a portion of the structural layer to define at least one microstructure, and removing the sacrificial layer such that the at least one microstructure is released from the substrate and the filler material forms at least one protrusion on the at least one microstructure.
These and other features and advantages of the invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the methods according to this invention.